Cryotropic hydrogels and their use as filters

ABSTRACT

There is provided a particle-filter comprising at least one hydrogel formed in aqueous solution at a temperature below 0° C. The filter can arrest particulates of smoke by physisorption on the solid surface of the gel or by chemisorption if the gel is suitably functionalized. The effectiveness of filter in arresting the solid particles and the pressure drop can be controlled by varying the porosity and the pore size distribution of the filter which in turn can be modulated by varying the monomer to water and monomer to cross-linker ratio in the pre-polymer solution. The filter is able to reduce the particle content of the smoke. There is further provided a cigarette comprising at least one particle-filter according as well as a method for the filtration of smoke.

This application is a continuation of U.S. patent application Ser. No. 13/256,976 filed on Mar. 9, 2012, which is a 371 of PCT/SE2010/050285 filed on Mar. 16, 2010 and claims priority to Sweden Application No. 0950156-0 filed on Mar. 16, 2007, which are hereby incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to the field of cryotropic hydrogels and their use for filters. The present invention further relates to the design of smoke filters including a cigarette smoke filter using crosslinked porous materials. The invention further concerns a method for the filtration of smoke.

BACKGROUND

Cigarettes have always been a topic of conflict between the health organizations and the cigarette companies because of harmful and carcinogenic compounds. In the 1950s, in the wake of rapidly growing scientific evidence that cigarettes cause lung cancer, companies tried to make special filters which would arrest the tar and other components from being inhaled with the smoke. Over the years several advancements have been made in regard to filter design. In general, the filters have been designed using fibers made of thermoplastic polymeric materials. For example U.S. Pat. No. 4,059,121 describes a filter element made of solid fibers oriented in randomly manner. The diameter of these fibers range up to 5 μm. While, this patent talks about a single filter element, U.S. Pat. No. 4,149,550 describes a filter core made of randomly oriented fibers and a crust with densely packed fibers. Thus along with the filtering efficiency, the strength of the porous structure of the filter too has been targeted to be enhanced by this procedure. While these filters are made of single polymeric material, U.S. Pat. No. 4,579,130 proposes to use two different types of crystalline polymeric materials which are mixed and extruded to form the fibers. The patent further proposes to enhance the mechanical strength of the filter by enhancing the adhesion between the fibers which are thermally and mechanically treated for this purpose. U.S. Pat. No. 4,869,275 describes the design of an entangled web of very low bulk density made of plastic fibers of micron size diameter. Filtration efficiency of as high as 80% is achieved in this filter. U.S. Pat. No. 4,961,415 describes the use of non-woven thermoplastic polypropylene fibers, but here, in addition, the filter consists of regions made of fused columnar structure which does not increase the pressure drop but impart mechanical strength to the filter. Similarly U.S. Pat. No. 5,538,019 envisages randomly oriented filaments formed into loop springs using which a desired degree of firmness and pressure drop is achieved. Filter of a different kind is proposed in U.S. Pat. No. 5,586,987 which depicts the design of a thermoplastic elastomer-based fibrous nonwoven web formed into a bag filter with an open, a close and inside surface. A bi-component sheath-core filter is envisaged by U.S. Pat. Nos. 5,633,082 and 6,026,819 which describe the use of thermoplastic material like polypropylene, polybutylene terephthalate and ethylene-vinyl acetate copolymer. These polymers in different composition are used to form the core and the sheath and a seamless continuity over these two regions. While the above patents desires essentially to achieve high filtration efficiency while maintaining the mechanical integrity of the filter, these are several other patents which have been proposed to improve the quality of smoke being inhaled, by the addition into the matrix of the filter of different components and molecules which either diffuse slowly into the smoke or arrest something toxic. For example, U.S. Pat. No. 5,115,823 proposes to impart flavor to the smoke by a novel design which comprises of a filtering zone and a flavor enhancing zone containing the flavor. Similarly, U.S. Pat. Nos. 5,746,231 and 6,164,288 propose dispersing of a humectant like sodium pyroglutamate, chlorophyllin, vegetable oil etc. into the filtering material which captures the moisture in the smoke. This moisture then helps in wet filtering of the smoke. U.S. Pat. No. 6,530,377 proposes to incorporate these molecules inside microcapsules dispersed through the matrix of the filter. Beside these humectants, there are other type of molecules that facilitate removal of toxic chemicals have been proposed. For example, U.S. Pat. No. 6,792,953 describes the use of cellulose fibers impregnated with metallic ions like copper and iron containing porphyrins. Similarly, U.S. Pat. No. 7,104,265 proposes addition of copper and iron phthalocyanine into the filter material. Beside the smoke quality, the disposal of the filter material has also been a targeted issue. For example, there are patents which propose to design biodegradable cigarette filters; U.S. Pat. No. 5,817,159 proposes to design an interpenetrating network made of two water soluble polymers. Importantly, the fibers for such filters are prepared without using any organic solvent. Similarly, U.S. Pat. No. 5,911,224 uses polyvinylalcohol as the fiber material which too is biodegradable. Since, polyvinylalcohol is hygroscopic, the patent describes also the special treatment procedure for carefully controlling the moisture of the fibers. While the above patents describe fibrous design of the filter, U.S. Pat. No. 5,360,023 envisages filter made of web of paper incorporating a carbonaceous material. The papers arranged in a way to provide longitudinally extending channels.

Details of preparation of cryogels have been dealt with in U.S. patent application Ser. Nos. 10/492,404 and 10/552,034. Previously cryogels have been used for variety of applications e.g. for chromatographic separation methods and as biomaterials. Another application has been proposed by U.S. Pat. Nos. 5,288,503 and 5,460,715 which have described respectively cryogel diffusion barrier for release of therapeutic agents and as a filtering medium for separating blood cells from plasma.

Whereas the existing filters have provided some hope for a safer cigarette smoking, the particle count in the inhaled smoke in traditional cigarettes (Capstan, ITC brand) is still very high: 10⁹ particles per centimeter cube of the smoke or higher. Thus there is room for improvement regarding cigarette filters.

SUMMARY

It is an object of the present invention to obviate at least some of the disadvantages of the prior art and provide an improved particle-filter as well as a method for the filtration of smoke.

It is described to use a polymeric material known as cryogel which have been found to reduce the particle count significantly (10⁷ particles per centimeter cube) without increasing the pressure drop any significantly.

There is provided a particle-filter comprising at least one hydrogel formed in aqueous solution at a temperature below 0° C.

There is further provided a cigarette comprising at least one particle-filter as described above.

There is also provided a method for the filtration of smoke comprising the steps:

providing at least one filter comprising a macroporous hydrogel formed in aqueous solution at a temperature below 0° C., and filtering the smoke through the filter.

Further aspects and embodiments are defined in the appended claims, which are specifically incorporated herein by reference.

Advantages of the invention include that:

The cigarette filter is prepared without using any organic solvent and the crosslinked network is not soluble in water in contrast to many other types of filter materials.

The filter material does not cause any harm to the environment or to the human body and is biodegradable.

The filter allows use in both dry condition or by incorporating an amount of water in it. While water enhances the arresting effect of the particulates that is the tar materials and other components in the smoke, it increases the pressure drop. Since the filter material is hydrophilic it contains the water in its pores without allowing the water to wet the surface of its container e.g. the paper wall of a conventional filter.

It is possible to achieve filtration via physical adsorption onto the surface of the cryogel material or by specific chemical reaction if the crosslinked network is suitably functionalized.

When the filtration occurs by physisorption, the filter can be recycled after simple washing with water.

The crosslinked structure provides mechanical strength to the filter without significantly increasing the pressure drop

The mechanical strength i.e. the deformability of the material can be tuned by controlling the pore size distribution of the filter.

The filter material is macroporous which helps in reducing the pressure drop.

The macroporous structure is tuned by varying the water to monomer ratio and the monomer to crosslinker ratio in the pre-polymer solution.

The filter can be used as a substitute of the conventional cigarette filters or can be used as an additional filter to the native cigarette filter.

DEFINITIONS

Before the invention is disclosed and described in detail, it is to be understood that this invention is not limited to particular compounds, configurations, method steps, substrates, and materials disclosed herein as such compounds, configurations, method steps, substrates, and materials may vary somewhat. It is also to be understood that the terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting since the scope of the present invention is limited only by the appended claims and equivalents thereof.

It must be noted that, as used in this specification and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.

If nothing else is defined, any terms and scientific terminology used herein are intended to have the meanings commonly understood by those of skill in the art to which this invention pertains.

“Cryogel” is used herein to denote a hydrogel formed in aqueous solution at subzero temperatures.

“Hydrogel” is used herein to denote a network of crosslinked water-soluble polymer chains.

“A macropourous” material is a material containing pores with an average diameter greater than 50 nm but not more than 50 μm, along with interconnectivity between the pores.

“Smoke” is used herein to denote a collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion or pyrolysis. For instance smoke often comprises particles comprising tar. Examples of substances in cigarette smoke include but are not limited to methane, acetylene, ethane, propene, chloromethane, propane, ethanol, acetaldehyde, butene, ethanol, acetonitrile, acrolein, acetone, acrylonitrile, isoprene, pentadiene, 2-butanone, hexane, benzene, dimethylfuran, pyridine, toluene, benzopyrine, napthalene, N-nitrosamine, chloro-biphenyls, phenols, 2-pentanone, 3-buten-2-one, 3-pentanone, ethylbenzene, isobutyronitrile, isoprene, o-xylene, propionaldehyde, propionitrile, m-/p-xylene, styrene and toluene.

“A supermacroporous” material is a material comprising pores greater than 1000 nm (1 μm) along with interconnectivity between the pores.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of this invention will be more apparent from the ensuing description when read in conjunction with the accompanying drawings wherein:

FIG. 1 shows scanning electron microscope image of a typical cryogel sample. The image shows the presence of pores and the pore wall made of the polymeric material. The pore size is a function of the polymer and the crosslinker concentration.

FIG. 2 shows schematic of the experimental set up for particle count analysis in the smoke emitted by a burning cigarette. The cigarette 21 is first attached to one end of a cylindrical holder 22 which contains also a smoke filter 23. This filter is either a conventional filter or one pre-designed of cryogel material. The SMPS 25 (Scanning Mobility Particle Sizer Spectrophotometer) sucks in the smoke 24 at constant flow rate which brings in the tar and other content from the burning cigarette and analyze it to yield the particle size distribution of the smoke 24.

FIG. 3 shows number density N_(p) of particles per cc volume of smoke is plotted against diameter D_(p) (nm) of particles. There is represented the particle count in smoke released respectively by a single (D) and double (C) conventional cigarette filter, and the particle count in smoke released by John silver cigarette filter (E). There is further shown particle count in smoke emitted by polyacrylamide cryogel filters prepared by using 6% (A) and 7% (B) by weight of monomer respectively. The monomer to crosslinker ratio is maintained at 2:1 for both the gels.

FIG. 4 shows number density N_(p) of particles per cc volume of smoke is plotted against diameter D_(p) (nm) of particles. There is represented the particle count in smoke released respectively by a single (H) and double (G) conventional cigarette filter, and the particle count in smoke released by John silver cigarette filter (I) as well as that emitted by agarose-alginate (2:1) cryogel filter (F).

FIG. 5 shows number density N_(p) of particles per cc volume in smoke released by the acrylamide cryogel filters prepared with varying monomer to cross-linker ratio plotted against the particle size D_(p) in nm. Here the monomer concentration is kept constant at 6%. J denotes a 3:1 Cryogel, K denotes a 4:1 Cryogel.

FIG. 6 shows pressure drop (Pa) across the filters plotted against the flow rate (cc/min) of gas through the filters. There is represented pressure drop across a conventional cigarette filters Capstan (L) and an auxiliary cigarette filter-John Silver (N). There is also represented pressure drop offered by cryogel filters made of Agarose-alginate (2:1) (M) and Acrylamide (6%, 3:1) (O) respectively.

FIG. 7 shows Number density N_(p) of particles per cc volume of smoke is plotted against diameter D_(p) (nm) of particles. Comparison of particle reduction by agarose-alginate cryogel filters in wet (Q) and dry (P) condition.

DETAILED DESCRIPTION

In a first aspect there is provided a particle-filter comprising at least one hydrogel formed in aqueous solution at a temperature below 0° C.

There is in one embodiment described a highly efficient particle-filter comprising a cross-linked inter-connected macroporous hydrogel structure to reduce particulate components in mainstream cigarette smoke.

There in one embodiment provided a cigarette filter which has been prepared by using crosslinked material e.g. “cryogel” synthesized using acrylamide, or N-isopropylacrylamide monomers but not excluding others. There are also described filters synthesized by cryogel matrices of synthetic polymers like agarose and/or alginate. These materials are prepared without using any organic solvent and are biodegradable. The filtration process is achieved via physical adsorption onto the surface of the cryogel material or by specific chemical reaction if the crosslinked network is suitably functionalized. The filter is recyclable and biodegradable. One advantage of the crosslinked structure is that it provides mechanical strength to the filter without significantly increasing the pressure drop. Furthermore, both these parameters can be tuned by controlling the pore size distribution. This cigarette filter has been found to reduce the content of particles comprising tar in the cigarette smoke by several orders of magnitude from what is achieved by the conventional filters used in cigarettes. The filter can be used in both wet and dry condition. The macroporous nature and interconnectivity makes these cryogel filters as highly efficient substitutes of the conventional cigarette filters or can be used as an additional filter to the native cigarette filter.

The invention discloses the application of porous crosslinked materials like cryogel as the base material for designing cigarette smoke filters. The cross linking polymerization is in one embodiment carried out inside a hollow solid cylinder in order to achieve a cylindrical structure (monolith) of the filter of specific dimensions to suit for application of use as filters.

In one embodiment the particle-filter is a cigarette filter.

In one embodiment the hydrogel of the particle-filter is insoluble in water.

In one embodiment the hydrogel of the particle-filter comprises water. This has the advantage that water arrests particles, for example particles comprising tar in cigarette smoke.

In one embodiment the hydrogel of the particle-filter is hydrophilic. This has the advantage that the filter contains water in its pores without allowing the water to wet the surface of its container, e.g. the paper wall of a conventional filter.

In one embodiment the hydrogel of the particle-filter is supermacroporous.

In one embodiment the hydrogel of the particle-filter comprises at least one material selected from the group consisting of poly-acrylamide, agarose, and alginate.

In one embodiment the hydrogel is formed at a temperature below −10° C. In one embodiment the hydrogel is formed at a temperature below the freezing point of the solution comprising the monomer. In an alternative embodiment the hydrogel is formed at a temperature 10 degrees Celsius below the freezing point of the solution comprising the monomer.

In one embodiment the hydrogel of the particle filter is formed without using any organic solvents. This has the advantage that it does not cause harm to the environment and the human body.

In a second aspect there is provided a cigarette comprising at least one particle-filter as described above.

In a third aspect there is provided a method for the filtration of smoke comprising the steps:

providing at least one filter comprising a macroporous hydrogel formed in aqueous solution at a temperature below 0° C., and filtering the smoke through the filter.

In one embodiment the at least one filter is recycled.

In one embodiment the at least one filter is washed with water, when it is recycled.

In one embodiment particulate matter in the smoke is reduced. In one embodiment the reduction of particulate matter is more than 95 wt %. In another embodiment the reduction of particulate matter is more than 99 wt %. In yet another embodiment the reduction of particulate matter is more than 99.9 wt %. In an alternative embodiment the reduction of particulate matter removes essentially all particles.

In one embodiment the amount of at least one substance in the smoke is reduced, wherein the at least one substance is at least one substance selected from the group consisting of methane, acetylene, ethane, propene, chloromethane, propane, ethanol, acetaldehyde, butene, ethanol, acetonitrile, acrolein, acetone, acrylonitrile, isoprene, pentadiene, 2-butanone, hexane, benzene, dimethylfuran, pyridine, toluene, benzopyrine, napthalene, N-nitrosamine, chloro-biphenyls, phenols, 2-pentanone, 3-buten-2-one, 3-pentanone, ethylbenzene, isobutyronitrile, isoprene, o-xylene, propionaldehyde, propionitrile, m-/p-xylene, styrene and toluene. In most cases the amount of several or all of the above substances in smoke are reduced. In one embodiment all of the above substances are reduced. The above substances are abundant in smoke such as smoke from cigarettes.

In one embodiment the amount of the above at least one substance in the smoke is reduced by more than 90 wt %. In a further embodiment the amount of the at least one substance in the smoke is reduced by more than 95 wt %. In another embodiment the amount of the at least one substance in the smoke is reduced by more than 99 wt %. In another embodiment the amount of the at least one substance in the smoke is reduced by more than 99.9 wt %.

The hydrogel is in an alternative embodiment made from at least one type of monomers or polymeric precursors selected from the group consisting of acrylamide, N-isopropylacrylamide, acrylonitrile, N-vinylcaprolactam, chitosan, gelatin, alginate, agarose and poly(vinylalcohol).

The cryogel is in one embodiment synthesized by using a polymerization reaction in the water medium under freezing conditions which yield a spongy, elastic and supermacroporous material. Normally the freezing conditions are below 0° C.

In general the filters according to the present invention are useful for filtering gases comprising particulate matter and substances in gas phase, for example the smoke generated by a cigarette, dust laden smoke in many industrial situations e.g. mines, blast furnaces, cement plants etc. Cryogel filters can be employed in disposable particulate respirators for filtration of aerosols and fine particulates suspended in air, which are associated with human disorders like asthma, lung cancer etc.

Other features and uses of the invention and their associated advantages will be evident to a person skilled in the art upon reading the description and the examples.

It is to be understood that this invention is not limited to the particular embodiments shown here. The following examples are provided for illustrative purposes and are not intended to limit the scope of the invention since the scope of the present invention is limited only by the appended claims and equivalents thereof.

EXAMPLES

The cryogels of poly(acryl amide) [poly(AAm)] were synthesized by radical polymerization at −12° C. during 12 hours using monomers of acryl amide (AAm) with N,N-ethylene bisacrylamide (MBAAm) as cross-linking agent. Similarly, agarose-alginate cryogels were fabricated by cross-linking agarose and alginate with the help of glutaraldehyde at −12° C. for 16 hours.

When the polymerization occurs at sub zero temperature the water freezes and form nucleated crystals which grow and prevents the polymer reaction from occurring inside the crystals of water. After the polymerization reaction is complete the gel is brought back to the normal temperature at which the crystals melt resulting in the porous structure. The sizes of these pores depend on the polymer concentration, cross-linker concentration and the rate of cooling. The moisture content of the filter is controlled by drying it to a desired extent.

The supermacroporous nature of these hydrogels was defined visually by scanning electron microcopy and ESEM (hydrogels in hydrated state). The quantitative estimation was done by mercury porosimetry, uptake of water and cyclohexane. The evaluated pore size is up to 200 μm.

The approach is to exploit the filtration properties of polymeric matrices called as “cryogels” to reduce the particulate count of smoke by using cryogels as auxiliary or substitute to conventional filters.

Cryogel filters have been used as auxiliary filters to observe reduction in total particle matter (TPM) in main stream cigarette smoke. Cigarette smoke coming out of the cigarette butt having a single conventional cigarette filter has about 10⁹ particles per centimeter cube (“D” in FIG. 3). These “cryogel filters” are macroporous materials having many advantages over the currently used cigarette filters as they remove significantly higher number of particles comprising tar compared to traditional auxiliary filters like John Silver or Capstan's cigarette filter used as auxiliary filter. As can be seen in FIG. 3, acrylamide cryogel (7% monomer, 2:1) as auxiliary filter reduces total particulate count by a factor of about 10⁴ in comparison to TPM of cigarette having its filter (“D” in FIG. 3). Even if an additional cigarette filter is used as auxiliary filter with the cigarette (“C”, FIG. 3) the TPM reduction is less in comparison to the reduction offered by cryogel filters. Similarly 6% acrylamide cryogel also reduces particulate count by a factor of 10³ to 10² when compared to auxiliary Capstan cigarette filter and auxiliary John Silver filter respectively (FIG. 3).

Agarose-alginate cryogel filter gives a particle reduction by a factor of 10⁴ in comparison to the particle reduction by traditional cigarette filter, when both of them were used as auxiliary filters (FIG. 4).

Moreover, the synthesis protocol of the cryogel filters can be varied to design cigarette filters with desired reduction in particulate count and optimum draw resistance. As the polymer concentration of the cryogel increases the particle count decreases e.g. the particle reduction offered by acrylamide cryogel filter with 7% (total monomer) concentration is higher than that offered by 6% acrylamide cryogel filter (FIG. 3). Similarly on decreasing the polymer to cross linker ratio in the cryogel the particle count passing through “cryogel filter” also decreases (FIG. 5). This change in porosity by varying the polymer concentration and the cross linker to polymer ratio gives the cryogel of different porosity which changes the total particulate count (TPM) in the smoke.

Along with particle count reduction, the draw resistance of the filters is also an important property. To characterize these cryogel filters for draw resistance, we measured the pressure drop across these filters when nitrogen gas was flowed through these cryogels at different flow rates and compared them with the pressure drop of capstan cigarette filter and John silver filter. FIG. 6 shows that both acrylamide and agarose-alginate cryogel filters have comparable pressure drop as with capstan cigarette filter, thus making them eligible candidates for use as cigarette filters. 

1. A method of manufacturing a particle-filter, said particle filter comprising at least one hydrogel, said method comprising the step of forming the hydrogel in aqueous solution at a temperature below 0° C., wherein said particle filter is dried to an essentially dry condition, wherein the dried particle filter is ready to use as a particle filter.
 2. The method according to claim 1, wherein said particle-filter is a cigarette filter.
 3. The method according to claim 1, wherein said particle-filter is insoluble in water.
 4. The method according to claim 1, wherein said particle-filter is at least partially enclosed by paper.
 5. The method according to claim 1, wherein said particle-filter is supermacroporous.
 6. The method according to claim 1, wherein said particle-filter comprises at least one material selected from the group consisting of poly-acrylamide, agarose, and alginate.
 7. The method according to claim 1, wherein said particle-filter is formed at a temperature below −10° C.
 8. The method according to claim 1, wherein said particle-filter is formed without using any organic solvents. 